add tests

but they fail why?

SockMerchant.sock_merchant import get_number_of_matching_pairs_of_socks
E   ModuleNotFoundError: No module named 'SockMerchant'
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
========================================================================================================================= 1 error in 0.07 seconds ==========================================================================================================================
(goshen) [kus@localhost goshen]$ pytest
=========================================================================================================================== test session starts ============================================================================================================================
platform linux -- Python 3.7.2, pytest-4.4.0, py-1.8.0, pluggy-0.9.0
rootdir: /home/kus/src/wyoming/experimental/python/goshen
collected 0 items / 1 errors

================================================================================================================================== ERRORS ==================================================================================================================================
_______________________________________________________________________________________________________________ ERROR collecting tests/test_sock_merchant.py _______________________________________________________________________________________________________________
ImportError while importing test module '/home/kus/src/wyoming/experimental/python/goshen/tests/test_sock_merchant.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
tests/test_sock_merchant.py:1: in <module>
    from sock_merchant import get_number_of_matching_pairs_of_socks
E   ModuleNotFoundError: No module named 'sock_merchant'
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
========================================================================================================================= 1 error in 0.08 seconds ==========================================================================================================================
(goshen) [kus@localhost goshen]$